1. Field of the Invention
The present invention relates to a method for heating or otherwise treating metal objects, and more particularly, to a method and apparatus for inductively heating or otherwise treating metal can lids (closures, ends) or can bodies for drying, curing or other purposes, for maintaining a spacing between them, and for motivating them along a path.
2. Description of Related Art
Closures for metal beverage containers are generally of a circular shape with a flanged perimeter called a curl. The closures are usually made of aluminum or steel, and the curl is used in attaching the closure to a can body through a seaming operation. To aid the integrity of the seal thus formed between the can body and the closure, it is a common practice to apply a bead of sealant or adhesive ("compound") within the curl during manufacture of the closure. Different types of coatings are also selectively or generally applied to can closures and can bodies for various other purposes as well, for example, to repair damaged coatings. For the purposes of the present description, coatings, sealants and adhesives are all considered to be "liquids" applied to a workpiece.
One problem which arises in this manufacturing operation is the curing or drying of such liquids. Recently there has been increased interest in the use of water-based sealants in the container industry, which may take 3-4 days to dry to an acceptable state for application of the closure to a can body. This was not a severe problem for solvent-based liquids, because the volatile solvent quickly evaporates and is acceptably dry for application of the closure to a can body typically within 48 hours.
In the past, can closures were heated to aid the drying or curing process typically either by infrared radiation or convection heating. These systems, especially the convection heating systems, tended to be large, bulky and expensive to operate due to inefficient energy usage.
Attempts have also been made to heat can ends inductively. Induction heating of electrically conductive articles involves passing an oscillating current through a work coil to create an oscillating magnetic field in which the electrically conductive workpiece is placed. Heat is produced in the workpiece as a result of eddy current losses resulting from circulating currents induced in the workpiece by the field.
FIG. 1 shows a typical electrical circuit for inductive heating. It comprises a DC power supply 110 with an H-bridge 130 connected across its outputs. The H-bridge 130 is made up of four thyristors 112, 114, 116 and 118, and coupled across the H-bridge is a parallel resonant tank circuit 120 in series with a current-limiting inductor 122. The tank circuit 120 includes the work coil 124, a load represented by an equivalent resistance 126, and a capacitor 128, all coupled in parallel.
In steady state operation, energy is coupled back and forth between the capacitor 128 and the inductor 124, resulting in an oscillating voltage across the tank circuit 120. In order to compensate for energy losses through equivalent resistance 126, control circuitry (not shown) periodically activates the thyristors of the H-bridge in order to pump additional energy into the tank circuit 120. The oscillating voltage has a frequency which varies somewhat depending on the load (i.e. workpiece) present in the heater at any given point in time, but prior systems have not taken this into account in determining when to activate the thyristors except possibly in a gross manner, applied over a long period of time.
The series inductor 122 is sometimes inserted to reduce the current peaks into the tank circuit 120 when the thyristors are activated, and to protect against large current surges in the event of malfunction. The current flow through the thyristors still can be enormous, however, requiring water (or liquid) cooling both of the thyristors themselves and of the wires coupling the power supply 110 to the H-bridge 130 and the H-bridge 130 to the tank circuit 120.
A typical induction heating circuit operating at high frequencies (on the order of 100 kHz) may use power MOSFETs as the switches in the H-bridge since MOSFETs can turn on and off very quickly. MOSFETs capable of operating at the higher voltages of induction heating circuits have a high on resistance, however, so they still need to be water cooled. Some induction heating circuits may use SCRs in the H-bridge, but although SCRs can turn on and off quickly, they are difficult to control. Standard bipolar transistors require large base current amplifiers to operate and do not work well at high switching speeds and at the high currents which the H-bridge must carry in the circuit of FIG. 1.
One problem with prior art inductive heating methods which might be used to heat can closures derives from the fact that in the past they have typically operated at high frequencies (on the order of 100 kHz), which tends to minimize the depth to which any currents are generated in the can closure. This meant that the oscillating currents in conductors traveling to and in the work coil also traveled primarily along the outside surface of these conductors (the "skin effect"). The current density along the outside surface of these conductors was therefore very high, causing excessive heating and necessitating water cooling. Typically, in fact, these conductors were constructed using copper tubing with water flowing through the center.
Heating of can ends by induction has been difficult also because they are made of sheet metal. Induction heating at high frequencies creates problems of non-uniform heating. Various portions of a sheet metal workpiece have been heated to greatly varying temperatures depending on proximity to the coil and other factors. Consequently, localized overheating can easily and frequently does occur, even before other parts of the workpiece are heated to a desired temperature. Collins U.S. Pat. No. 4,017,704 attempts to solve this problem by placing metal can ends flat on a conveyor belt and passing them under an ordinary few-turn, high-frequency induction heating coil having a generally open center area, and a bow tie-shaped core disposed adjacent to the center of the coil in order to focus the energy more uniformly into the can ends. Even if Collins' technique were successful for heating can ends uniformly, however, the technique still requires focusing cores and water-cooled wires (copper tubing) and switches.
In addition to Collins, induction heating apparatus in general, although not necessarily addressing the special problems of can ends and can bodies, is also shown in the following U.S. Pat. Nos.: 4,339,645 to Miller, 4,481,397 to Maurice, 4,296,294 to Beckert, 4,849,598 to Nozaki, 4,160,891 to Scheffler, 3,449,539 to Scheffler, 4,307,276 to Kurata, 4,582,972 to Curtin, 4,673,781 to Nuns, 4,531,037 to Camus, 4,775,772 to Chaboseau, 4,810,843 to Wicker, and 3,727,982 to Itoh. While some of the systems disclosed in these references may be usable for heating can closures, they are not optimal. In particular, for example, they may be very large and bulky, may require water cooling, and may suffer from non-uniform heating when applied to can ends.
Metal can closures are typically conveyed into the heat-treating apparatus in either of two ways. They can be conveyed by a conveyor belt, in which case the closures lie flat on the belt with coating or compound side up, or they can be stacked within a track or cage, in abutting face-to-face contact with each other ("in-stick"). The former technique is exemplified in the Collins patent. In the latter technique the closures are pushed through the apparatus in a direction transverse to their faces. Heat treating of can ends being pushed through in-stick would require less floor space since many more can ends can be packed into a given length of track. The technique is not often used, however, because convection air currents cannot heat the faces of the can ends directly.
Sullivan U.S. Pat. No. 4,333,246 attempts to address this problem, but still within the confines of convective drying techniques. In Sullivan, the workpieces are pushed through a curvilinear path defined by a constant width trackwork, allowed to pivot on the portions of the workpieces in proximity to the shorter radiuses whereby fan-like separation of the portions in proximity to the longer radius occurs. Sullivan uses this trackwork to partially separate can lids as heated air is directed toward the separated portions.
The Sullivan technique has a number of major disadvantages. First, though one portion of each of the workpieces is separated from the other workpieces, there is always another portion of the workpieces (the portions in proximity to the shorter radiuses) which are touching other workpieces. The pieces are only fanned, not truly separated. Thus, if the apparatus is being used to cure liquids applied selectively on can lids, for example, it can be used only where the selectively applied liquid has been applied somewhere other than around the circumference where the lids are likely to touch each other. Additionally, the pressure on the portions of the lids which do touch each other, caused by the forces pushing the lids along the track, can soften and/or damage the metal of the lids or their coating. Moreover, the Sullivan apparatus can generate only limited separation between the fanned portions of the can lids, since greater separation requires tighter curves in the trackwork, which in turn requires greater force and stronger materials in the equipment which pushes the lids along the track. Nor can the technique be used for long conveyance paths, for the same reason, even if the curves are kept shallow. Still further, Sullivan's technique will not work well with can lids which have pull rings, since these can lids do not nest well and are likely to scratch each other if they touch.
Whether can ends are transported flat or in-stick, the conveyance velocity and the length of the drying apparatus are chosen to ensure that a sufficient amount of the water or solvent in the applied liquid has been driven out by the time each can closure emerges from the apparatus. A problem arises, however, if the production line should stop for some reason or somehow become blocked. In this case, the can closures in the heating apparatus would remain there longer than originally intended, thereby overheating them and potentially destroying them. Closed-loop mechanisms have been provided in the past for handling this situation, but these mechanisms have only monitored air temperature in the furnace. No closed loop mechanism has monitored the temperature of the can closures themselves. Furthermore, for IR systems and high-temperature convection systems, where mechanisms were provided it was difficult to stop the heating process quickly enough to avoid damage. Lower temperature convection heating systems do exist which avoid the risk of overheating can lids simply because they never get hot enough to cause damage, but the lower temperatures undesirably also necessitate longer drying times and longer conveyance paths.